Are climate models biased?

Not really, but they may exaggerate the impact of volcanic eruptions.

If you took an average output of multiple climate models, it would predict that the start of this century would have seen a strong warming trend. Instead, the planet warmed relatively slowly over this time.

When models and reality disagree, it can tell us about two things: the models and reality. So far, analysis has seemed to come down on the side of reality. Evidence has indicated that one of the contributors to this century's climate has been small volcanic eruptions; another suggests that a run of La Niña years has helped hold temperatures down.

Now, a new study is out that turns the focus on the models. It finds no evidence that the models are biased toward predicting higher temperatures and instead suggests that their biggest issue might be in how they handle large volcanic eruptions.

The work, performed by Jochem Marotzke and Piers Forster, looks for gaps between reality and the models in two ways. The first is to simply run an ensemble of climate models (the CMIP5 collection) and use them to generate 15-year trends for the global surface temperature, with each year getting its own trend value. Because the models won't experience chaotic events, like El Niño, in synchrony, the range of values they produce provides a measure of the natural variability captured by the models.

Over the century-plus from 1900 to 2012, the actual temperature trends are all entirely within this range of natural variability. There are times (most of the years between 1910 and 1935, for example) when the real-world temperatures were at the upper edge of the model collection. At other times, the actual temperature trends ended up near the bottom of the range seen in the models—one of those times being the periods that started in the '90s and stretched into this century. So while the current warming is on the low side of the models, it's still within the range of natural variability.

The authors found that there's no systematic bias throughout the range; that is, the models aren't consistently running warmer or cooler than reality. So they searched for periods in which reality was closest to the edge of the models' natural variability. The three periods they identify are all associated with large volcanic eruptions. Thus, they conclude that the models may slightly exaggerate the cooling driven by eruptions and then have a corresponding overreaction when the warming rebounds later.

To find out what 15-year trends may tell us about the climate in general, the authors performed a multiple regression analysis, which attempts to identify which of a list of factors are having the largest impact on a trend (in this case, the temperature trend). When calculated for 15-year trends, the analysis pointed the finger at natural variability being the largest influence. When the calculations were extended to 62-year trends, however, changes in radiative forcings—aerosols, greenhouse gasses, and solar output—became the dominant factors.

This is another way of saying that if you're only looking at 15-year-long periods, you're mostly going to be seeing the impact of natural variations internal to the climate system. To see the influence of greenhouse gasses, you have to extend your view. The implications for what the temperature trends of this century tell us should be obvious. In fact, the authors say that the models are good for the entire period for which there's a solid instrumental record: "viewed over the entire period 1900–2012, no systematic model error needs to be invoked when trying to explain differences between simulated and observed trends."

By only doing 15- and 62-year trends, the authors don't pinpoint where the crossover between the influence of internal variability and the influence of greenhouse gasses might take place. But the fact that internal variability is over twice as strong as the next most important factor at 15 years suggests that it's not going to be at 16 years. Until they get much more computer time, however, that's probably all we're going to be able to say.